You will need an array to store the letters that have been guessed. The easiest way would be to make an array element for each letter of the alphabet and have a boolean value for if it has been guessed or not.
I will put question marks where I think you can work out what to put there.
// I can't remember how to define types so maybe this is pseudo code
type guessType
char as string
guessed as boolean
endtype
dim letter(25) as guessType
for i = 0 to 25
letter(i).char = ? // If you are stuck look up ASCII.
letter(i).guessed = 0
next i

Formerly OBese87.